1. Field of the Invention
The present invention relates to a pneumatic tire in whose tread portion a white filler is used, and more particular, to technology for improving the produceability of pneumatic tires.
2. Description of the Related Art
In recent years, research has been carried out on reducing the rolling resistance of tires in order to reduce the fuel costs for automobiles as one of the demands made by society for the conservation of energy and the conservation of resources. The rolling resistance of a tire can be reduced by lowering the heat build-up property of the tire. This may lead to a decrease in the temperature of a tire which is being traveled on, and may improve the durability of a tire, especially a large-sized tire.
It is well known that using a rubber composition having low hysteresis loss as a tread rubber is good for decreasing the heat build-up property of a tire. However, when a rubber composition having low hysteresis loss is used as a tread rubber, the road surface gripping properties, particularly the gripping properties on wet road surfaces, tend to deteriorate, which is not preferable from the standpoint of safety. There is thus the demand for a tire which exhibits both a good low heat build-up property and good gripping properties.
However, in most cases, rubber compositions in which a white filler is compounded have a slow vulcanization speed, which results in problems with regard to vulcanization produceability. In particular, when attempts are made to use such a rubber composition in the tread of a pneumatic tire, it is often the case that the tread portion is the thickest gauge portion of an unvulcanized tire, and the vulcanization produceability is often determined by the vulcanization speed of this portion. Therefore, the vulcanization produceability is sacrificed. Further, when attempts are made to carry out vulcanization sufficiently all the way through to the inner portion of the tread, there arises the problem that, due to the poor thermal conductivity, the surface of the tread is overcured and the properties thus deteriorate.
A rubber composition, in which natural rubber or butadiene rubber is the main rubber component, is often used in particular in tires in which a low heat build-up property is important and in large tires in which durability is important. However, the properties deteriorate greatly when such rubber compositions are overcured. A tire tread rubber composition which has a low heat build-up property, a fast vulcanization speed, and little deterioration of properties when overcured has been desired.
Various studies have been conducted with regard to these problems. Rubber Chemistry and Technology, Vol. 67, pp. 672-686 discusses an improvement in the properties of a rubber composition due to the combined use of silica and a thiophosphoryl compound. However, there is no disclosure therein of the specific metal salt which is used in the present invention, no disclosure relating to application to tires, and no suggestion nor study of properties required of tires such as those described above, i.e., low heat build-up property, high gripping performance, good produceability, and the like.
Further, U.S. Pat. No. 3,426,003 discloses the use of zinc dithiophosphate and a thiuram compound as a combination of vulcanizing accelerators for butyl rubber. However, in U.S. Pat. No. 3,426,003, natural rubber or butadiene rubber is not the main rubber component, and there is no disclosure of a white filler which is essential in the present invention.
British Patent No. 1,288,616 discloses improving discoloration by the combined use of zinc dithiophosphate and a white filler, but does not include any study nor suggestion of the various properties required of a tire such as those described above.
Japanese Patent Application (JP-A) No. 47-9488 discloses a method of vulcanizing a rubber compound formed from a diene-based rubber component, a specific diradical dithiophosphate, a known vulcanization accelerator, and sulfur. The obtained vulcanizate has a superior resistance to heat aging. However, this rubber compound includes substantially no white filler such as silica. Further, no studies have been carried out on tires using such a rubber compound. JP-A-47-9488 includes no recitations relating to, nor even any suggestion of, the properties required of a tire such as described above.